Many types of implantable medical devices have been clinically implanted into patient's bodies over the last twenty years that deliver relatively high-energy cardioversion and/or defibrillation shocks to a patient's heart when a malignant tachyarrhythmia, e.g., atrial or ventricular fibrillation, is detected. Cardioversion shocks are delivered in synchrony with a detected R-wave when fibrillation detection criteria are met, whereas defibrillation shocks are delivered when fibrillation criteria are met and an R-wave cannot be discerned from the electrocardiogram. The earliest clinically released automatic implantable defibrillators (AIDs) that were implanted in human patients provided a high energy defibrillation shock developed by an AID implantable pulse generator (IPG) through a pair of epicardial electrodes applied directly to the epicardium of the heart exposed through a thoracotomy when high heart rate detection criteria were met. Later developed and clinically implanted implantable cardiodefibrillators (ICDs), originally referred to as pacemaker/cardioverter/defibrillators (PCDs), possessed more sophisticated detection algorithms and provided defibrillation, R-wave synchronized cardioversion, and pacing therapies to treat a variety of malignant tachyarrhythmias ranging from fibrillation to fast tachycardias. Current ICDs typically additionally possess single or dual chamber bradycardia pacing capabilities for treating specified chronic or episodic atrial and/or ventricular bradycardia and tachycardia. The most current clinically released ICDs also include right and left heart chamber pacing capabilities for improving the cardiac output of patient's hearts that are in heart failure. Unless otherwise indicated, all of the above-described IMDs are referred to herein as ICDs.
It was postulated early in the development of ICDs that cardioversion/defibrillation shocks could be delivered between large surface area patch electrodes implanted subcutaneously over the ribcage on either side of the heart as indicated in the article by Schuder et al. entitled “Experimental Ventricular Defibrillation with an Automatic and Completely Implanted System”, Transactions American Society for Artificial Internal Organs, 16:207, 1970.
Subcutaneous leads are implanted between the patient's skin and rib cage with a tunneling tool. Conventional subcutaneous implantable leads are implanted by first forming a subcutaneous channel with a tunneling tool that is coaxial with a sheath. After the channel is formed, the tool is removed leaving the sheath disposed in the channel. The lead is subsequently threaded down the sheath into a desired subcutaneous position. The sheath is thereafter removed by withdrawing and slitting the sheath to bring the sheath over a proximal end of the lead. Withdrawing the sheath from the channel can undesirably snag the lead and move it from is desired subcutaneous position.
It is desirable to provide implantable leads for use with ICDs that are simpler to place subcutaneously.